(1) Field of the Invention
The present invention relates to a magnetic shielding material for a television (TV) cathode-ray tube, particularly for a color TV cathode-ray tube, and more particularly to a material of a magnetic shielding component disposed on the interior or exterior of the cathode-ray tube to surround an electron beam in a funnel of the tube, that is, a magnetic shielding material for a TV cathode-ray tube, and a process for producing the same.
(2) Description of the Prior Art
A color TV cathode-ray tube basically comprises an electron gun and a fluorescent screen for converting an electron beam to an image and, further, a magnetic shielding component, for preventing the electron beam from being deflected by geomagnetism, surrounding an inside of the funnel of the cathode-ray tube. This magnetic shielding component is known also as an inner shielding component or an inner magnetic shielding component. In some cases, an outer shielding component for shielding geomagnetism on the exterior of the cathode-ray tube is used. In the present invention, the materials for these components are generally called "magnetic shielding materials."
The magnetic shielding material is generally a steel sheet having a thickness of 0.1 to 0.5 mm, and a coil of this steel sheet is pressed by an electric appliance manufacturer and then incorporated into the interior of the cathode-ray tube. In order to shield geomagnetism, it is common practice to use a method wherein a degaussing coil wound around the exterior of the color cathode-ray tube is energized by an alternating current and an anti-magnetic field is created by the residual magnetism in the interior of the inner shield component to reduce the effect of external geomagnetism.
The general properties required of the magnetic shielding material are a high magnetic permeability .mu., in the geomagnetic field (a very low strength magnetic field of about 0.3 Oe (Oe is oersted)), necessary for facilitating passage of magnetism, and a low coercive force Hc for improving the degaussing properties (elimination of residual magnetism). In order to ensure the above magnetic properties of the magnetic shielding material, in Japanese Unexamined Patent Publication (Kokai) No. 2-282423, aluminum (not more than 0.001%) as the constituent of steel is minimized, and, at the same time, boron is added so that the fine precipitate in the steel is brought to substantially BN alone, thereby attempting to improve the magnetic permeability and to lower the coercive force and, at the same time, to improve the press moldability.
Similarly, in order to improve the magnetic properties, moldability, spot weldability, graphitizability and other properties of the magnetic shielding material, Japanese Unexamined Patent Publication (Kokai) No. 3-146644 discloses a technique, where the total amount of silicon, manganese, and aluminum is specified and a given amount of boron is added, and describes that, in this case, a coercive force of not more than 1 Oe at a magnetic field of 10 Oe is desired.
Further, Japanese Examined Patent Publication (Kokoku) No. 5-64698 discloses an inner shielding material wherein, in order to improve the magnetic properties of the inner shielding material for a TV cathode-ray tube, the grain size and residual strain are regulated to provide an inner shielding material having a magnetic permeability of not less than 750 emu and a coercive force of not more than 1.2 Oe (maximum magnetizing force 10 Oe) at a d.c. magnetic field of 0.3 Oe.
In recent years, however, the size and the width of domestic TVs have been markedly increased, lengthening the travel distance and scanning distance of an electron beam and increasing the migration of the electron beam deflected by the geomagnetism. This is causative of color shading. A demand for personal computers has been rapidly increased, and since the image on the monitor of the personal computers is a static image and viewed at short range, what is particularly required is a landing property of a high precision electron beam.
That is, at the present time, there is a strong demand, in TV cathode-ray tubes, for a magnetic shielding material which can reduce the drift of electron beam landing.